Vinyl acetate is produced commercially by the catalyzed partial oxidation of ethylene in the presence of acetic acid and oxygen. The oxygen source may be commercially available oxygen or air. Generally, in an oxygen-based process, ethylene, acetic acid and oxygen are mixed with a recycle gas and fed to the reactor. The reactor comprises a number of tubes which are placed inside a vessel and arranged similar to shell and tube heat exchangers. The reactor tubes are filled with preferably a metallic catalyst on a porous support containing small amounts of promoters. A coolant circulates in the shell around the reactor tubes to maintain temperature control.
In an oxygen-based process, a typical composition of the gas stream that is fed to the reactor tubes includes 40 to 60 mol % ethylene, 5 to 10 mol % oxygen, 4 to 10 mol % argon, 10 to 15 mol % acetic acid, 5 to 15 mol % carbon dioxide, with ethane, nitrogen and water constituting the remainder of the composition.
Ethylene and acetic acid react with oxygen to form vinyl acetate and also in a side reaction to form carbon dioxide and water. Both reactions are exothermic. The reactor effluent is treated in two separate steps by removing product vinyl acetate and remaining reactant acetic acid, and by removing byproduct carbon dioxide. The remaining gas is recycled after a portion of it is purged. The purge stream is required in order to keep impurities in the reactor at acceptable levels. Impurities, like argon, are introduced in the oxygen stream, and like ethane or propane, in the ethylene feed stream. A significant amount of ethylene is lost in the purge stream as a selectivity loss. Typically, a purge gas composition is 65.0 mol % ethylene, 7.0 mol % oxygen, 5.0 mol % argon, 17.8 mol % carbon dioxide, 4.0 mol % nitrogen, and the remaining being ethane and methane.
In general, argon impurities introduced with the oxygen stream determine the size of the purge stream when oxygen concentration of the oxygen feed is between 98 mol % and 99.6 mol %. If the amount of argon introduced in the reactor decreases, then the size of the purge stream can be lowered and ethylene loses can be reduced. An oxygen feed with higher oxygen concentration (&gt;99.6 mol %) will result in reduction of the purge volumetric flow rate if argon is the impurity that controls the purge. However, other impurities are also introduced in the process. For example, because of the corrosive nature of the acetic acid in the process, the instruments that are used for process control require nitrogen purge/blowback. Nitrogen is the inert gas of choice and ends up in the recycle. Nitrogen must therefore be removed with the purge to prevent nitrogen from building up in the recycle. If nitrogen enters the reactor in amounts similar to those of argon, then steps must be taken to reduce the nitrogen concentration before reducing the argon concentration.
Various methods for treating the purge from oxygen-based reactions to recover ethylene have been proposed. For example, U.S. Pat. No. 4,904,807 discloses the use of an argon selective membrane that is used to treat the purge and separate it into two streams 1) an argon rich stream that is vented and 2) an ethylene rich stream that can be recycled back to the ethylene oxide reactor, and U.S. Pat. No. 4,769,047 discloses the use of pressure swing adsorption to remove ethylene from the purge and recycle it back to the reactor. A major disadvantage in these methods is the large capital cost of the associated equipment.
It is believed that there has not been a commercially practical solution to reduce the impurities associated with the production of vinyl acetate. Therefore, there is a need to provide a new method for producing vinyl acetate which maximizes the selectivity and minimizes ethylene loses to purge, thus improving the yield of the vinyl acetate production.